Forza Motorsport 6: Apex

The fastest way I've found to rack up race points is to keep repeating the Raptor Strike event in the Tours of Motorsport. It's the 4th event and it involves driving a Ford Raptor truck and knocking over bowling pins, It takes about a minute and a half to complete including loading time to restart the event once you complete it. If you turn all assists off and drive fast completing all 3 bonus objectives, you can easily get over 1 million race points per attempt and just keep restarting and redoing it until you get to 50 million.

If you're going for the rest of the achievements, you will get race points completing all the other events, so I wouldn't worry about grinding this particular challenge until you complete the others. I had about 39 million once I completed all the other achievements. But this challenge is quick and easy for finishing up this achievement.

Here's a video of the event. It's for the other achievement for getting a platinum medal but this is what you do over and over for this achievement. You do not have to return to the main menu after each race. You can just hit restart and keep repeating it.

1 million points in 2 minutes - with automatic shifting and racing line (if you want.)

Event
Tours of Motorsport #4 - "Raptor Strike"

Assists
Suggested Line: Braking only (Enough to show you where to go)
Braking: ABS off (Not needed with this car)
Steering: Normal
Traction & Stability Control: Off (Not needed with this car)
Shifting: Automatic
Damage, Fuel & Tire Wear: Simulation (It's virtually impossible to damage the car here)
Rewind: Off (Not needed and a waste of time)

Drivatar difficulty is not relevant for this event, since there are no other drivers.

I recommend the hood camera for best visibility. But use whatever you feel most comfortable with.

On the back straight, aim for the pins on the right side to avoid aquaplaning through the big puddle on the left.

After finishing the race, select "Restart" to avoid loading times.

You can either go fast and hit only the pins in your way, or you can go slower and hit more pins - it doesn't make much difference, due to the time bonus. Either way, after a couple of runs you should be scoring 1 million points in about 1:20 minutes consistently. Simply repeat until the achievement pops.

The easiest way to get this is to go onto the 4th race on the career/story mode and then turn all the assists to the hardest, I was left with a percentage bonus of 69% as I used a manual without clutch! From there simply go through this race over and over as it's not too difficult. It will be challenging at first however once you have done it a few times you'll be getting 1,100,000/1,200,000 every 2 minutes.

TIPS: Go slowly around as the time challenge isn't an issue, there is no need to try and go too quickly, you can also skip a load of pins like I do in the video and still comfortably get 1,100,000 each time!

After an hour or so the achievement will unlock when you have got a career total of 50,000,000 :)

For this achievement Data Collection and Telemetry has to be turned on (automatic) to enable progress tracking and to unlock (for most of the other achievements it as to be turned off). It finally popped when reaching 58 million racepoints (ingame).